Brown growth on mouth and back

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The guppies and neon tetras in my tank have a brown fungus-like growth on their mouths and some guppies have it on their back ...

The guppies and neon tetras in my tank have a brown fungus-like growth on their mouths and some guppies have it on their back also. At first I thought this might just have been the normal color for the guppies but now that I've seen it on the neons too I've become suspicious. The fish act normal besides an occasional flick. Any ideas on what it could be? If I didn't describe it enough I will try to get a picture if necessary.

The only thing I can find that is similar to my problem is Cotton Mouth Disease. However cotton mouth is normally white and what my fish have is a dark brown. I don't know what it is, there is nothing about it on any website for fish diseases. Unless it is an odd colored cotton mouth. I don't know what to do.
I think it is a fungus even though I can't find any listed illnesses anywhere that match it.

Here's an update since no one seems to know what it is. I replaced my purigen a few days ago and now this stuff is clearing up. I'm still puzzled as to what it is though. If I had to guess I would say my purigen was used up and wasn't filtering everything out and it started to build up on the guppies somehow, but that theory is most likely wrong; that explanation just seems to simple.